The present invention relates to a heat pressure-fixable capsulated toner for use in the development of electrostatic latent images formed in electrophotography, electrostatic printing or electrostatic recording, and a process for the production thereof.
Conventional electrophotographies comprises a developing step which comprises of uniformly electrifying a photoconductive insulating layer, subjecting the layer to exposure, dissipating the charge on the exposed portion to form an electrical latent image, and then depositing a fine charged powder having a coloring agent (referred to as a toner hereinbelow) to form a visual image, a step of transferring the resultant visual image onto a transfer material such as transfer paper, and a step of permanently fixing the visual image by heating, pressure or other suitable fixation procedure.
In general, a toner is subjected to a variety of mechanical stresses caused by rolling action of a developing roller and a toner-supplying roller during operation in a developing apparatus, and mechanically deteriorates for a long time of the operation. It has been known that the use of resins having a large molecular weight can generally reduce the deterioration of the toner. However, since it is necessary to heat a heat-roller to a high temperature for the satisfactory fixation of the resins, the use thereof causes some problems, such as enlargement of fixation apparatus, curl of papers, large energy consumption and prompted deterioration of the fixation apparatus.
In order to solve the problems mentioned above, a proposal has been made on the use of a capsulated toner comprising a core material and an outer shell which covers the surface of the core.
This proposal intends to obtain a capsulated toner having both of good fixation and good blocking resistance by a combination of the core made of easily-fixable and thermally soft materials, or having a low melting point and the outer shell made of comparatively hard material having a good blocking resistance. The term "blocking" used herein means to form an agglomerate of toner particles adhering to each other.
Some proposals have been made on a variety of techniques using a wax having a low melting temperature such as a liquid wax and a rubber-like wax at a room temperature, as a core material. A shell material can be generally selected from hard materials. When the shell material is soft and has a low strength, although a fixation is improved, it would be difficult to obtain the intended property of toner since the shell material would separate from the core material and the toner itself would deform due to a stress caused by a developing apparatus.
When the shell material is hard and has a high strength, the fixation of the toner becomes bad since crushing the shell requires a large pressure and/or a high temperature. Consequently, a proposal has been made on a capsulated toner for heat roller fixation comprising a resin having a low glass-transition temperature, as a core material, which causes a blocking at a high temperature when it is used alone, but which improves the fixation of the toner for heat-pressure fixation, and a resin having a high glass-transition temperature as a shell material, which is useful for increasing a blocking resistance of the toner.
The conventional techniques mentioned above have the following problems to be solved. Although there has been a lot of the proposed toner having an outer shell and a core, which is produced with a variety of materials and by a variety of processes, the toner is not sufficient in fixation at a low temperature, offset resistance, blocking resistance and endurance. That is, an ideal process for the production of toner having a core-shell structure has not been established yet. The term "offset" used herein means that a toner melts at an elevated temperature to adhere to a fixation roller in a fixation step.
Among the above-listed properties of toner, it is very difficult to coincidentally satisfy the fixation at a low temperature and the blocking resistance in the toner. Taking a printing apparatus, in particular a contact-developing system, into consideration, these properties should be balanced in highly good condition. The reasons for the balance is as follows:
Since a development in the contact developing system is done by adding a high pressure between a developing roller and a photoreceptor drum, the toner thin layer formed on the developing roller is subjected to an intense stress.
The toner subjected to such a high pressure easily deteriorates at an elevated temperature to cause troubles, for example, fixing onto a surface of photoreceptor drum. It causes troubles in printing. Although such troubles can be prevented by employing a resin having a high glass-transition temperature or adding a large amount of shell material to produce the toner, the fixation at a low temperature will be lost to lower an ability of the toner.
In other words, a fixation at a low temperature is inconsistent with a blocking resistance in a toner, in particular the toner that for a contact developing system. In view of the inconsistency, it would be very important to produce a capsulated toner practically useful for a contact developing system.
As mentioned above, the toner conventionally proposed has not satisfied a balance of a fixation at a low temperature and a blocking resistance, and has not been practically useful for contact developing at a high pressure. This is because a heat characteristic of core resin of the toner was designed to enhance a fixation property by using a resin having a low glass-transition temperature.
Consequently, it has been desired to develop a pressure-fixable toner for a heat pressure-fixation system using a heat roller, which is excellent in an offset resistance, a fixation at a low temperature, a blocking resistance and an endurance in a long time operation, in particular, a capsulated toner practically useful practically useful in a contact development system.